The present invention relates generally to the field of fuel burners and in particular to a new and useful air separation cone for expanding the internal recirculation zone near the exit of one or more air zones surrounding a fuel delivery nozzle.
Low-NOx fossil fuel burners operate on the principle of controlled separation and mixing of fuel and oxidizer for minimizing the oxidation of fuel-bound nitrogen and nitrogen in the air to NOx (i.e., NO+NO2). Use of overfire air in conjunction with fuel-rich combustion is referred to as external (or air) staging. Internal staging involves the creation of fuel-rich and fuel-lean combustion zones within the burner flame. With proper design, fuel-air mixing and swirl patterns can be optimized to create a reverse flow region or “internal recirculation zone” (IRZ) near the burner exit for recycling heat and combustion products including NOx from fuel-lean regions into fuel-rich zones to sustain ignition, maintain flame stability, and convert NOx to N2. Both internal and external staging are often necessary for maximum NOx reduction. Flames with large, high temperature, sub-stoichiometric (oxygen-deficient) IRZ's generally produce very low NOx levels since such conditions are conducive for NOx destruction. Low-NOx burner designs produce the IRZ by imparting swirl on the air and/or fuel streams as well as flow deflecting devices such as flame holders and air separation cones.
FIG. 1 shows a low-NOx pulverized coal fired burner 900 having a conventional air separation cone. Primary air and pulverized coal 902 are blown into an inlet and pass through a burner elbow 904. The pulverized coal concentrates along the outer radius at the elbow exit. The pulverized coal enters the inlet end of a fuel nozzle or tubular burner nozzle 906, and encounters a deflector 908 which redirects the coal stream into a conical diffuser 912, which disperses the majority of the pulverized coal particles entrained in the primary air to a location near the inside surface of the tubular burner nozzle 906, leaving the central portion of the nozzle 906 relatively free of pulverized coal particles.
Secondary air 910, or the majority of combustion air, is delivered to inner and outer secondary air zones 914 and 916 from the burner windbox. Swirl can be imparted into the zones 914 and 916 via adjustable angle spin vanes 922 in the inner air zone 914 and both fixed spin vanes 920 and adjustable angle spin vanes 922 in the outer air zone 916. The inner and outer secondary air zones 914 and 916 are formed by concentrically surrounding walls. The inner air zone 914 concentrically surrounds the tubular burner nozzle 906 and the outer air zone 916 concentrically surrounds the inner air zone 914.
An air separation cone 924, concentrically surrounding the end of the tubular burner nozzle 906, helps channel the secondary air 910 leaving the inner and outer air zones 914 and 916. A flame stabilizer 926 and a slide damper 928 control the secondary air 910. The flame stabilizer 926 is mounted at the end of the tubular burner nozzle 906 while the air separation cone 924 is installed on a cylindrical sleeve that separates the inner and outer secondary air zones 914 and 916.
The inner and outer zones 914 and 916 direct the secondary air radially outward by the combined action of the burner throat and the swirl imparted by the spin vanes 922, generating internal recirculation zones (IRZ) 930. FIG. 1 shows the predicted reverse flow IRZ streamlines for a low-NOx pulverized coal fired burner 900 having a conventional air separation cone 924. NOx is formed along the outer air-rich periphery of the flame as secondary air is introduced from the inner and outer air zones. The IRZ causes the NOx formed at the outer fringe of the flame to recirculate back along the fuel rich flame core, where hydrocarbon radicals react to reduce the NOx.
The size of the IRZ can be increased somewhat by imparting more swirl on the secondary air flow, and extending the flow deflection devices, or increasing their angle of attack. Generation of high swirling flows require fan power boosting due to higher pressure drop. High swirl combustion can also intensify the fuel/oxidizer mixing and generate high NOx emissions. Extension of flow deflecting devices (flame holder or air separation cone) into the furnace could expose those parts to high flame temperatures and cause damage. Increasing the angle of attack on the flow deflecting devices could restrict the air flow passages, raise the pressure drop, and diminish the swirl effects. Therefore, a device is needed for safely and effectively increasing the size of the IRZ, without damaging flow deflecting devices, causing increased NOx emissions, or raising pressure drop.